Best Practices for Writing Test Code: A Comprehensive Guide

When delivering an application, the codebase is typically divided into two main types: production code and test code. Production code implements the application logic, while test code focuses on automated tests. While numerous guides exist on writing good production code, test code often doesn't receive the same attention, leading to maintenance issues and unreliable tests.

This article explores good practices for writing test code, highlighting its similarities and differences with production code and how they complement each other.

Serving Business Requirements

Both production and test code serve business requirements. Misalignment can lead to overengineered, hard-to-maintain code. To ensure consistency:

- Align with Business Scenarios: Tests should verify that business scenarios are covered, not just that the implementation works.

- Consistency: Follow established coding style guidelines for tests. Use linters and static analysis tools optimized for test files.

- Readable Test Descriptions: Write test descriptions as use cases (e.g., "highlight error in form when entered phone number contains letters") rather than implementation states (e.g., "emit error to form for invalid input").

- Return on Investment: Focus on critical business scenarios. Evaluate the effort and payoff of testing edge cases.

Test Scope and Separation of Concerns

Minimizing test scope and following the principle of separation of concerns helps maintain test reliability:

- Focus on Specific Use Cases: Provide tests with only the necessary information and interactions.

- Tailored Mocks: Create minimal mocks for low-level tests and avoid sharing mocks across unit tests.

- Test Pyramid: Prioritize unit and integration tests over E2E tests to maintain efficiency and reduce maintenance costs.

Example:

Unit Test: Validate the `isValidPhoneNumber` function.

Integration Test: Ensure the phone number is stored in the database after an API request.

UI Component Tests: Check form validation and submission behavior.

E2E Test: Simulate the entire workflow of adding a new phone number.

Test Design and Maintenance

Maintaining test code involves ensuring it is readable, maintainable, and reliable:

- Descriptive Naming: Use clear and descriptive names for all entities in the test code.

- Readable Tests: Avoid excessive comments. Well-written code should be self-explanatory.

- Valid Tests: Ensure tests pass or fail based on meeting business criteria.

- Regular Review: Periodically review existing tests to ensure they still align with the current production code.

Test Setup

Organizing and documenting test setup is crucial for accessibility and consistency:

- Distinguish Test Suites: Separate unit tests, integration tests, and others with specific commands.

- Documentation: Include a basic guide on running tests in the project's README.

- Reusable Utilities: Extract and document reusable utilities, framework configurations, and mocking procedures.

- Leverage Tools: Use well-supported tools or libraries instead of building custom solutions.

Test Architecture

Reliable and efficient test execution requires careful architectural considerations:

- Predictable Behavior: Ensure tests run consistently across different environments.

- Isolation: Each test should start and end with a clean state. Use setup and teardown hooks effectively.

- Parallelism: Introduce parallelism to avoid bottlenecks as the application grows.

- Testability: Write production code with testability in mind, using mocking and spying techniques.

Simplicity in Test Code

The DAMP principle (Descriptive And Meaningful Phrases) emphasizes simplicity in test code:

- Repetition: Unlike production code, test code can benefit from repetition to enhance readability.

- Hardcoded Values: Use explicit values in unit tests for clarity.

- Realistic Values: Use values that reflect real application scenarios.

- Linear Control Flow: Avoid conditionals and loops in tests to maintain straightforward, linear test logic.

Example:

Simple Tests: Provide mocks directly in tests and use explicit values. Each test should focus on a single use case.

Bottom Line

Test code and production code complement each other. While test code indicates areas needing improvement in production code, it also has unique challenges. Test code is primarily read and debugged, often by those who did not write the original functionality. Therefore, maintaining simplicity and readability in test code is crucial. By implementing straightforward, valuable tests and focusing on linear test flows, you can enhance test efficiency and maintainability, ultimately saving time and ensuring reliability.